Referring to FIGS. 1 and 2, the operational principle of contact film printers consists of running a film negative 4 in contact with an unexposed film raw stock (R/S) 3 over a film sprocket (or printing sprocket) 29 and exposing the R/S with a light source 16 which illuminates the R/S through the negative 4. The raw stock 3 and the negative 4 are held in close contact against the printing sprocket 29 by a gate roller 19. The sprocket engages spaced apart perforations along both sides of the film.
Many existing contact printers use a printing sprocket 29 having a single full fitting sprocket 1 on one side and a free running ring 2 on the other side, as shown in FIG. 1. The full fitting sprocket has sprocket teeth 7 which are approximately as wide as the film perforation width 11. Both the printing sprocket and the free running ring have separate rotating shafts and are independently mounted on ball bearings 9 aligned on a common axis of rotation 15.
As shown in FIG. 3, some film printers use a dual printing sprocket 30 comprised of a full fitting sprocket 3- and a shrinkage sprocket 6. (In this case, the free running ring 2 shown in FIG. 1 has been replaced by the shrinkage sprocket 6 shown in FIG. 3.) The full fitting sprocket has sprocket teeth 7a and the shrinkage sprocket has sprocket teeth 7b. The tooth width 12 of the full fitting sprocket is shown in FIG. 3. The tooth width 13 of the shrinkage sprocket 6 is substantially narrower than the width 11 of the film perforation. (The narrower width is intended to compensate for film shrinkage or expansion.) The full fitting sprocket 1 and the shrinkage sprocket 6 are tied together on a common shaft 8 shown in FIG. 3. This design is referred to as a dual sprocket design.
Both the single sprocket and dual sprocket printers have problems with picture steadiness. Picture unsteadiness is introduced into the print when the negative 4 moves relative to the raw stock 3 during exposure. (The print is the developed or processed R/S on which the image is visible.) With the single sprocket, the steadiness problem is more severe on the free running ring side than on the sprocket side. To ensure a steady picture, the perforations 10 of both the negative and the R/S are tensioned against at least one sprocket tooth 7 of the printing sprocket. In the case of a dual sprocket, at least one perforation 10 on each side of the film must be tensioned against at least one sprocket tooth on the full fitting sprocket 1 and on one sprocket tooth on the shrinkage sprocket 6.
Prior art film printers have used spring tensioning rollers 5 shown in FIG. 2 to provide tension to the negative and the R/S; however, if the film becomes misaligned the tensioning system does not automatically correct for such misalignment because the rollers are fixed in position except for lateral movement allowed for by their tension spring. Even small misalignments of one or two thousandths of an inch can ultimately produce undesired jitter on the screen since the film is magnified up to about 100 times with current motion picture projection equipment.
Accordingly, there is a long felt need for a way to improve film processing by e curing the problems associated with film feed misalignment.